1. Field of Invention
The present invention relates generally to electrostatic latent image development, and more particularly, to an apparatus and method for developing an electrostatic latent image directly from an imaging member to a final substrate, such as paper or the like.
2. Description of Related Art
In a typical electrostatographic printing process, copying and printing are initiated by selectively charging and/or discharging a charge receptive imaging member in accordance with an original input document or an imaging signal, thereby generating an electrostatic latent image on the imaging member. This latent image is subsequently developed into a visible image by a process in which charged developing material is deposited onto the surface of the latent image bearing imaging member. Charged particles in the developing material adhere to image areas of the latent image to form a visible developed image corresponding to the latent image on the imaging member. The developed image is subsequently transferred, either directly or indirectly, from the imaging member to a copy substrate, such as paper or the like, to produce a "hard copy" output document. In a final step, the imaging member is cleaned to remove any charge and/or residual developing material therefrom in preparation for a subsequent image forming cycle.
The developing material typically comprises carrier granules having toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically attracted from the carrier granules to the latent image areas to create a powder toner image on the imaging member. Alternatively, the developing material may comprise a liquid developing material comprising a carrier liquid having pigmented marking particles (or "toner solids") and charge director materials dispersed and/or dissolved therein, wherein the liquid developing material is applied to the latent image bearing imaging member with the marking particles being attracted to the image areas of the latent image to form a developed liquid image. Regardless of the type of developing material employed, the toner or marking particles of the developing material are uniformly charged and are electrostatically attracted to the latent image.
The above-described electrostatographic printing process is well known and has been implemented in various forms in the marketplace to facilitate, for example, light lens copying of an original document, as well as printing of electronically generated or digitally stored images where the electrostatic latent image is formed via a modulated laser beam. Analogous processes also exist in other electrostatic printing applications such as, for example, ionographic printing and reproduction where charge is deposited in image-wise configuration on a dielectric charge retentive surface (see, for example, U.S. Pat. Nos. 4,267,556 and 4,885,220, among numerous other patents and publications), as well as other electrostatic printing systems wherein a charge carrying medium is adapted to carry an electrostatic latent image. It will be understood that the instant invention applies to all various types of electrostatic printing systems and is not intended to be limited by the manner in which the image is formed on the imaging member or the nature of the latent image bearing member itself.
The typical electrostatographic printing process described above includes a development step whereby developing material is physically transported into contact with the imaging member so as to selectively adhere to the latent image areas thereon in an image-wise configuration. Development of the latent image is usually accomplished by electrical attraction of toner or marking particles to the image areas of the latent image. The development process is most effectively accomplished when the particles carry electrical charges opposite in polarity to the latent image charges, with the amount of toner or marking particles attracted to the latent image being proportional to the electrical field associated with the image areas. Some electrostatic imaging systems operate in a manner wherein the latent image includes charged image areas for attracting developer material (referred to as charged area development (CAD), or "write white" systems), while other printing processes operate in a manner such that discharged areas attract developing material (referred to as discharged area development (DAD), or "write black" systems).
U.S. patent applications Ser. No. 08/883,292 (the application Ser. No. '292), filed Jun. 27, 1997, and Ser. No. 08/884,236, (the application Ser. No. '236) filed Jun. 27, 1997, each herein incorporated by reference in its entirety, disclose electrostatographic imaging processes where an electrostatic latent image bearing member having a layer of marking material coated thereon is selectively charged in an image-wise manner to create a secondary latent image corresponding to the electrostatic latent image on the imaging member.
In the incorporated application Ser. No. '292, the image-wise charging is accomplished by a wide beam charge source for introducing free mobile charges, or ions, in the vicinity of the electrostatic latent image coated with the layer of marking material or toner particles. The latent image causes the free mobile ions to flow in an image-wise ion stream corresponding to the latent image. These ions, in turn, are captured by the marking material or toner particles, leading to image-wise charging of the marking material or toner particles with the layer of marking material or toner particles itself becoming the latent image carrier. The latent image carrying toner layer is subsequently developed by selectively separating and transferring image areas of the toner layer to a copy substrate for producing an output document.
In the incorporated application Ser. No. '236, the image-wise charging is accomplished by inducing the ionization of air via a phenomenon known as air breakdown for introducing free mobile ions in the vicinity of the electrostatic latent image coated with the layer of toner particles. The formation of electrostatic charge patterns by electrical discharges involves the phenomena of ionic conduction through gases. It is known that when two conductors are held near each other with a voltage applied between the two, electrical discharge will occur as the voltage is increased to the point of air breakdown. This discharge is usually accompanied by a visible spark. However, when the conductors are very close together (a few thousands of an inch) discharge can take place without sparking and electrical charges will be collected on a receiving surface during discharges. As described above, the latent image causes the free mobile ions to flow in an image-wise ion stream corresponding to the latent image. The ions, in turn, are captured by the marking material in the layer, leading to image-wise charging of the marking layer with the marking material layer itself becoming the latent image carrier. The latent image carrying toner layer is subsequently developed by selectively separating image areas of the toner layer and transferring the separated image to a copy substrate for producing an output document.
U.S. Pat. No. 4,504,138 to Kuehnle et al. discloses a method of developing a latent electrostatic charge image formed on a photoconductor surface. The method includes (1) applying a thin viscous layer of electrically charged toner particles to an applicator roller, preferably by electrically assisted separation thereof from a liquid toner suspension, (2) defining a restricted passage between the applicator roller and the photoconductor surface which approximates the thickness of the viscous layer, and (3) transferring the toner particles from the applicator roller at the photoconductor surface due to the preferential adherence thereof to the photoconductor surface under the dominant influence of the electric field strength of the electrostatic latent image carried by the photoconductive surface. The quantity of toner particles that is transferred is proportional to the relative incremental field strength of the latent electrostatic image. An apparatus for carrying out the method includes an applicator roller mounted for rotation in a container for toner suspension, an electrode arranged adjacent the circumferential surface of the roller to define an electrodeposition chamber therebetween and electrical connections between the roller, the electrode and a voltage source to enable electrolytic separation of toner particles in the chamber, forming a thin highly viscous layer of concentrated toner particles on the roller.
U.S. Pat. No. 5,387,760 to Miyazawa et al. describes a wet development apparatus for use in a recording machine to develop a toner image corresponding to an electrostatic latent image on an electrostatic latent image carrier. The apparatus includes a development roller disposed in contact with or near the electrostatic latent image carrier and an application head for applying a uniform layer of the wet developer to the roller.
U.S. Pat. No. 5,436,706 to Landa et al. discloses an imaging apparatus including a first member having a first surface with a latent electrostatic image formed thereon, wherein the latent electrostatic image includes image regions at a first voltage and background regions at a second voltage. A second member charged to a third voltage between the first and second voltages is also provided, having a second surface adapted for resilient engagement with the first surface. A third member is provided, adapted for resilient contact with the second surface in a transfer region. The imaging apparatus also includes an apparatus for supplying liquid toner to the transfer region thereby forming on the second surface a thin layer of liquid toner containing a relatively high concentration of charged toner particles, as well as an apparatus for developing the latent image by selectively transferring portions of the layer of liquid toner from the second surface to the first surface.
U.S. Pat. No. 5,619,313 to Domoto et al. describes a method and apparatus for simultaneously developing and transferring a liquid toner image. The method includes (1) moving a photoreceptor including a charge bearing surface having a first electrical potential, (2) applying a uniform layer of charge having a second electrical potential onto the charge bearing surface, and (3) dissipating charge, image-wise, from selected portions on the charge bearing surface to form a latent image electrostatically, such that the charge dissipated portions of the charge bearing surface have the first electrical potential of the charge bearing surface. The method also includes moving an intermediate transfer member biased to a third electrical potential that lies between said first and said second potentials, into a nip forming relationship with the moving imaging member to form a process nip. The method further includes introducing charged liquid toner having a fourth electrical potential into the process nip, such that the liquid toner sandwiched within the nip simultaneously develops image portions of the latent image onto the intermediate transfer member, and background portions of the latent image onto the charge bearing surface of the photoreceptor.
The liquid toner image development systems of the prior art that apply a substantially uniform layer of liquid toner to an imaging member do not contemplate developing an image directly onto a substrate at the same station where the image is separated from the imaging member. Rather, these prior art development systems develop and transfer the image from the imaging member to an image separator at a first station and subsequently transfer the image from the image separator to a substrate at a different station.